Extrapair paternity seems to be common in socially monogamous passerines, but the genetic mating system of most species is currently unknown. Here, we report the first study of paternity in the socially monogamous Common Crossbill (Loxia curvirostra). We found no evidence of extrapair paternity among 96 offspring in 34 examined broods. An upper 95% confidence limit of 3.1% suggests that extrapair fertilizations were truly infrequent in our study population. Common Crossbills thus seem to represent an exception to the rule of extrapair mating among socially monogamous passerine bird species. A potentially important selective pressure preventing promiscuity in Common Crossbills is the harsh environmental conditions experienced during breeding at wintertime, which may increase the importance of paternal care and limit the time available for seeking extrapair copulations.